Terephthalic acid is commercially produced by oxidation of paraxylene in the presence of at least one catalyst, such as, for example, Co, Mn, and Br catalyst and a solvent, typically acetic acid. Terephthalic acid is typically made in a manner to remove impurities formed as a result of the oxidation of paraxylene.
Terephthalic acid (TPA) is an intermediate in the production of condensation polymers and copolymers especially polyesters and co-polyesters for plastics, fibers, films, coatings, containers, and other articles. Of particular commercial importance is poly(ethylene terephthalate), referred to as PET, a polyester of TPA and ethylene glycol (EG), as well as related copolyesters. Commercial processes for the manufacture of TPA are often based on the multi-valent transition metal catalyzed oxidation of p-xylene, generally with a bromide promoter in an acetic acid solvent. Due to the limited solubility of TPA in acetic acid under practical oxidation conditions, a slurry of crystalline agglomerate containing primarily TPA is usually formed in the oxidation reactor. Typically, the TPA oxidizer slurry is withdrawn from the reactor, and TPA solids are separated from the oxidizer mother liquor using conventional solid-liquid separation techniques. The oxidizer mother liquor stream, which contains most of the catalyst and promoter used in the process, is recycled to the oxidation reactor. In addition to the catalyst and promoter, the oxidizer mother liquor stream also contains dissolved TPA and many by-products, impurities, and other compounds. These other compounds, oxidation by-products and impurities arise partially from compounds present in minor amounts in the p-xylene feed stream. Other compounds and oxidation by-products arise due to the incomplete oxidation of p-xylene resulting in partially oxidized products. Still other compounds and oxidation by-products result from competing side reactions formed as a result of the oxidation of p-xylene to terephthalic acid. Patents disclosing the production of terephthalic acid such as U.S. Pat. Nos. 4,158,738 and 3,996,271 are hereby incorporated by reference in their entirety to the extent that they do not contradict statements herein.
Many of the compounds in the oxidizer mother liquor stream that are recycled are relatively inert to further oxidation, but are not inert to further reaction including decomposition and conversion to other compounds. Such compounds include, for example, isophthalic acid (IPA), benzoic acid, and phthalic acid. Compounds in the oxidizer mother liquor stream, which may undergo further oxidation are also present, such as, for example in the case of oxidation of p-xylene (also known as 1,4-dimethylbenzene), compounds such as 4-carboxybenzaldehyde, p-toluic acid, p-tolualdehyde and terephthaldehyde. Compounds that are relatively inert to oxidation and are not otherwise removed from the process tend to accumulate in the oxidizer mother liquor stream upon recycle.
Conventionally, crude terephthalic acid (CTA) is purified either by conversion to a dimethyl ester or by dissolution in water with subsequent hydrogenation over standard hydrogenation catalysts. More recently, secondary oxidative treatments instead of hydrogenation have been used to produce polymer-grade TPA. It is desirable to minimize the concentration of impurities in the mother liquor and thereby facilitate subsequent purification of TPA. In some cases, it is not possible to produce a purified, polymer-grade TPA unless some means for removing impurities from the oxidizer mother liquor stream is utilized.
One technique for impurity removal commonly used in the chemical processing industry is to draw out or “purge” some portion of the mother liquor stream as a recycle stream. Typically, the purge stream is simply disposed of or, if economically justified, subjected to various treatments to remove undesired impurities while recovering valuable components. One example of this purge process is U.S. Pat. No. 4,939,297 herein incorporated by reference in its entirety to the extent that it does not contradict statements herein.
The purification of CTA to produce purified terephthalic acid (PTA) increases the manufacturing cost of the PTA. It is desirable to maximize the concentration of by-products, impurities, and other compounds in the terephthalic acid to the extent that the terephthalic acid remains useful, especially in making poly(ethylene terephthalate) (PET) polymer and articles therefrom, such as, film, containers, and fiber.
One example of utility is the improved yield in a carboxylic acid process, particularly a terephthalic acid process. Another utility of this invention is the flexibility of controlling the destination of specific compounds in the process. For example, a portion of specific compounds can be retained on the product in a catalyst removal zone, and or enriched in the product in the enrichment zones such that they go out with the product stream, or are allowed to exit the process. Yet another utility is the process allows the option of placing compounds on the product stream that are not in the TPA process. Another utility is the option of adding a comonomer, to the TPA product stream, for example, IPA, can be added.